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Growing Mushrooms: Sterilizing with Hydrated Lime

Happy 2017 folks! I hope you all enjoyed as much of 2016 as you possibly could; I know for certain that it was quite the year for me. Lots of changes were lingering underfoot that managed to push their way to the surface. I have been extraordinarily busy the last 9 months, as is evident due to the lack of posts that I have been writing, although I’ll have you know that I would have much preferred to have had the time to do so.

These changes have mostly been tied to my personal life as I now find myself in a promising new relationship in a brand new place; Winnipeg, located on the eastern fringes of the Canadian prairies. Here I hope to continue cultivating and sharing my enthusiasm and accumulated knowledge of the natural world; all the while taking the time to adjust to an unfamiliar psychological and ecological landscape.

Anyhow, It has therefore been extraordinarily slow on the blog front, considering my last entry was back in March of last year (2016) in which I discussed how you can go about harvesting, chopping, drying and roasting dandelion roots with the intent of making dandelion ‘coffee’. It turned out better than I expected, and if you are interested in that as a concept then I would recommend considering trying it out for yourself.

Despite the lull in productivity, the search for knowledge nevertheless creeps forward and I came across a technique for preparing (i.e. pasteurizing) substrate for mushroom growing that I previously overlooked, much to my own misfortune, that I knew I had to tell all of you current and future mushroom growers out there. So without further ado let us begin..

Many mushroom growers prefer to pasteurize or sterilize their substrates (the material that is used as a food source by the growing mycelium) using heat, often in the form of hot water (180 degrees F) or steam. The main drawback to this technique is that it requires quite a lot of energy on the cultivator’s end in order to heat up and maintain the temperature of the water or steam. To a certain extent, this method often requires special equipment to be purchased in order to make the procedure consistently effective.

An alternative to using heat pasteurization/sterilization is to use a compound called hydrated lime (calcium hydroxide) which is created when limestone (calcium carbonate) is burned. This substance is quite potent and can effectively sterilize even at a highly dilute concentration. One recipe courtesy of the American Mycological Society recommends dissolving approximately 4 cups (roughly 1 litre) in a 45 gallon barrel of water. This is sufficient to treat up to 20kg of substrate material!

It seems to me that almost any loose, easily handled material could be sterilized using this method, although chopped straw seems to be a favourite for many reasons mainly due to it’s low cost and availability in addition to it’s ease of handling. For the average home grower, a more reasonable ratio may be to use 110ml of hydrated lime dissolved in 5 gallons of water. Your substrate can then be soaked and submerged in the lime water for between 2-4 hours (4-6 hours also comes recommended).

You may also be wondering how calcium hydroxide works as a pasteurizer. Well, hydrated lime is an incredibly basic substance (in terms of pH, as in the opposite of acid) and therefore works by rapidly exposing the dormant spores and micro-organisms on the substrate to a solution which is in the range of pH 12 or 13. A soak lasting a few hours is sufficient to kill off possible contaminants through this violent change in pH.

After soaking, allow the substrate to drain on a perforated surface such as laying it on a wire rack or plastic drip tray for a few minutes before immediately mixing your spawn into the substrate and proceeding accordingly with your preparation. As always, ensure that the surfaces the substrate is coming into contact with are adequately clean, and this includes your hands. Don’t go overboard but try to be diligent, attentive, and plan ahead so you’re not scrambling.
And that’s all there is to the procedure, although there are a number of similar products out there that may sound or appear to be similar to calcium hydroxide, and so you definitely want to do your research to make sure that you are getting exactly what you (and your mushrooms) want. For example, pure calcium carbonate lime (also known as chalk or marble or derived from egg or oyster shells) is used as a substrate amendment after pasteurization in order to buffer the natural rise in acidity that occurs in the substrate during colonization by the mycelium.

Gypsum (calcium sulphate) is another compound that is often added to substrate to improve it’s texture and as a source of metabolic sulphur but, like calcium carbonate, it will not work as a sterilizer. Alternatively, adding dry hydrated lime (calcium hydroxide) to your substrate before mixing in your spawn will greatly harm the growing mycelium; this is why draining the substrate after it has soaked in the highly diluted lime water is an essential step in the process.

Lastly, hydrated lime that is made from dolomite, also known as dolomite lime or builder’s lime, has a high magnesium content and will not work as a sterilizer or amendment as the high magnesium levels will stunt the growth of mycelium. Therefore, when you are looking for the correct hydrated lime to use, ensure that it has a very high calcium content above 90% and that the magnesium content does not exceed 10%.
With all of this in mind, I highly encourage to you to try out this method the next time you are in the mood to start your next (or first) batch of mushrooms and if you do, let me know how it works. There are quite a few sources out there (including this one) that have reported higher yields (up to 150% biological efficiency with oyster mushrooms) using a soak in hydrated lime as their sole pasteurization method and report very few if any instances of contamination. Happy growing!

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4 comments

The global mushroom market is segmented on the basis of commonly produced types, into paddy, reishi or lingzhi, winter, oyster, shiitake, and button. Each type holds a greater prominence in different regions of the world, while a lot of them are prominently available in developed economies due to high demand.

There are a number of ways to sterilize sawdust. The most popular method is with a pressure cooker. The sawdust can be placed in polyethylene bags or glass jars and pressure cooked @ 15 PSI for an hour or more, depending on the density and weight of the bags.

Oyster mushrooms (Pleurotus spp., excluding the king oyster Pleurotus eryngii) can handle pasteurized sawdust, which is a process which can be completed without a pressure cooker. For pasteurization, the sawdust is exposed to either a hot water bath or is injected with steam that’s around 165 – 170 degrees F for several hours. The sawdust can then be drained, mixed with spawn, packed into bags / pails etc. and fruited.

There are lots of videos out there that show different ways of pasteurizing and sterilizing sawdust and provide specific time frames (eg. how long is needed to pasteurized 5 lb bags, 10 lb bags, etc.) so I encourage you to search around and see what makes the most sense to you.